In the manufacture of plastic injection molded products, plastic in a liquid form is transferred to a mold through an injection cylinder. The liquid plastic is typically advanced through the cylinder by an injection screw and is transferred to the mold through a nozzle tip connected to the end of the cylinder.
The end surface of the cylinder forms a sealing seat with the nozzle tip. In order to prevent the liquid plastic from leaking between the interface of the cylinder and the nozzle tip, the cylinder seat must be precisely flat. Over time the cylinder seat may become worn resulting in an unreliable seal. Once the cylinder seat becomes worn, it may be necessary to resurface it. In high production output operations, it is very expensive to shut down the system in order to resurface or recondition the cylinder seat. Typically, the injection cylinder must be disassembled and then transferred to another facility capable of resurfacing the injection cylinder seat. Additional costs usually arise by having to transport the injection cylinder to and from the other facility.
Lapping devices are known in the art for finish-grinding a surface. For example, U.S. Pat. No. 5,259,144 discloses a lapping device for a safety relief valve. This device comprises a plurality of grinding disks driven by a planetary gear system. Each disk rotates about its own axis and all of the disks are driven about a common central axis to lap the safety relief valve. The grinding face of each disk is in constant engagement with entire width of the surface of the valve seat during the grinding process. This is detrimental because it causes the edges of the valve seat to become rounded which creates a less effective seal.
U.S. Pat. Nos. 1,941,918; 2,984,054; 2,649,669; and 1,634,745 all disclose various lapping devices and grinding machines. However, all of these devices are complicated in operation and are expensive to manufacture. Additionally, these devices are limited in their application to only one type and size of valve or other workpiece and are not capable of precision grinding a seal face of various sizes of injection molding cylinders.